Switch circuits having integrated overdrive protection and related transmit/receive circuits and MMIC amplifiers

ABSTRACT

Monolithic microwave integrated circuits are provided that include a substrate, a transmit/receive selection device that is formed on the substrate, a high power amplifier formed on the substrate and coupled to a first RF port of the transmit/receive selection device, a low noise amplifier formed on the substrate and coupled to a second RF port of the transmit/receive selection device and a protection circuit that is coupled to a first control port of the transmit/receive selection device.

STATEMENT OF U.S. GOVERNMENT INTEREST

This invention was made with Government support under Contract No.11-D-5309 awarded by the Department of Defense. The Government hascertain rights in the invention.

FIELD

The inventive concepts described herein relate to microelectronicdevices and, more particularly, to microelectronic devices havingoverdrive protection.

BACKGROUND

Most radio frequency (“RF”) electronic devices may be damaged or evendestroyed if subjected to high RF signal levels. In many cases, thedamage may be caused by high current levels flowing through the RFelectronic device, which can result in overheating that may damage ordestroy circuits within the device. The damaging RF signal levels maycome from a variety of sources, some of which may be unpredictable, suchas lightning strikes or RF signals transmitted by other nearbytransmitters. Additionally, RF electronic devices may be damaged ordestroyed when subjected to over-voltage conditions (i.e., voltagelevels are applied to the device that are higher than the voltage thatthe device is rated to handle). This is particularly true with RFelectronic devices formed from materials such as gallium arsenide(“GaAs”) or indium phosphide (“InP”) based semiconductor materials thathave relatively lower critical breakdown fields as compared to, forexample, devices formed from wide-bandgap materials such as galliumnitride (“GaN”) based semiconductor materials which have much highercritical breakdown fields.

One example of an RF electronic device that may be susceptible to damagewhen exposed to high RF signal levels is a low noise amplifier. As isknown in the art, a low noise amplifier is an amplifier circuit includedin many RF communications systems that is used to amplify a received RFsignal before the received RF signal is passed to a receiver. Typically,the power level of RF signals that are passed to a low noise amplifierare relatively low, as the RF signals have typically been transmittedover an air interface and hence have been subjected to free spacelosses, which may be quite high in many cases. Low noise amplifiers maybe susceptible to damage if an RF signal having a high power level isinput thereto.

SUMMARY

Pursuant to some embodiments of the present invention, monolithicmicrowave integrated circuits are provided that include a substrate, atransmit/receive selection device that is formed on the substrate, ahigh power amplifier formed on the substrate and coupled to a first RFport of the transmit/receive selection device, a low noise amplifierformed on the substrate and coupled to a second RF port of thetransmit/receive selection device and a protection circuit (e.g., anoverdrive protection circuit) that is coupled to a first control port ofthe transmit/receive selection device.

In some embodiments, the transmit/receive selection device may be atransmit/receive switch such as, for example, a single-pole double-throwswitch. In other embodiments, the transmit/receive selection device maybe a circulator

In some embodiments, the monolithic microwave integrated circuit mayfurther include a sampling circuit that is configured to couple a sampleof an RF signal received at a third RF port of the transmit/receiveselection device and to provide the sample of the RF signal to theprotection circuit. The sampling circuit may be configurable to sampleat least one of a first amount and a second amount of the RF signal,where the second amount is greater than the first amount. The samplingcircuit may be a capacitor that is coupled to the third RF port. In someembodiments, the sampling circuit may further include a variablecapacitor.

In some embodiments, the protection circuit may include an RF powerdetector and a control signal generator circuit that is responsive tothe RF power detector, where an output of the control signal generatorcircuit is coupled to the first control port of the transmit/receiveselection device.

The control signal generator may include a transistor that switchesbetween an on-state and an off-state when an output of the RF powerdetector reaches a threshold level.

In some embodiments, the transmit/receive switch may comprise a firsttransistor that is coupled between a first reference voltage and a firstRF transmission path that extends between the first RF port and thethird RF port, a second transistor that is coupled between a secondreference voltage and a second RF transmission path that extends betweenthe second RF port and the third RF port, and a third transistor that iscoupled between a third reference voltage and a first node connectingthe first RF transmission path to the second RF transmission path.

In some embodiments, the protection circuit may be configured toincrease reflection of RF signals received at the third RF port inresponse to detecting that a magnitude of the RF signal exceeds apredetermined threshold.

In some embodiments, the protection circuit may be configured toreconfigure the transmit/receive selection device in response todetecting that a level of an RF signal at the third RF port exceeds afirst threshold during transmit operations and to reconfigure thetransmit/receive selection device in response to detecting that thelevel of the RF signal exceeds a second threshold during receiveoperations, the second threshold being less than the first threshold.

Pursuant to further embodiments of the present invention, circuits areprovided that include a substrate having a switch formed thereon thatincludes first through third RF ports, a first RF transmission pathcoupled between the third RF port and the first RF port, a second RFtransmission path coupled between the third RF port and the second RFport, a first transistor that is coupled between the first RFtransmission path and a first reference voltage, a second transistorthat is coupled between the second RF transmission path and a secondreference voltage, a third transistor that is coupled between a firstnode that connects the first and second RF transmission paths and athird reference voltage, and a protection circuit that is configured toturn on the third transistor in response to a level of an RF signal atthe third port exceeding a preset threshold.

In some embodiments, the switch comprises a single-pole double-throwswitch, the circuit further includes a high power amplifier coupled tothe first RF port and a low noise amplifier coupled to the second RFport.

In some embodiments, the circuit further includes a sampling circuitthat is configured to couple a sample of an RF signal received at thethird RF port and to provide the sample of the RF signal to theprotection circuit. The sampling circuit may comprise a capacitorvoltage divider network that is coupled to the third RF port. Thecapacitor voltage divider network may include a variable capacitor insome embodiments.

In some embodiments, the protection circuit may include an RF powerdetector and a control signal generator circuit responsive to the RFpower detector, where an output of the control signal generator circuitis coupled to a gate of the third transistor. In some embodiments, thecontrol signal generator circuit may include a transistor that switchesbetween an on-state and an off-state when an output of the RF powerdetector reaches a threshold level.

In some embodiments, the protection circuit may be configured toincrease reflection of RF signals received at the third RF port inresponse to detecting that the level of the RF signal exceeds a presetthreshold.

In some embodiments, a dissipative load may be provided between thethird transistor and the third reference voltage.

Pursuant to additional embodiments of the present invention,transmit/receive switches are provided that include a first RF port, asecond RF port, a third RF port that may be selectively connected to oneof the first RF port and the second RF port, a first RF transmissionpath coupled between the third RF port and the first RF port, a secondRF transmission path coupled between the third RF port and the second RFport, a first transistor that is coupled between the first RFtransmission path and a first reference voltage, a second transistorthat is coupled between the second RF transmission path and a secondreference voltage, and a third transistor that is coupled between afirst node that connects the first and second RF transmission paths anda third reference voltage.

In some embodiments, the transmit/receive switch further includes aprotection circuit that is configured to turn on the third transistor inresponse to a level of an RF signal at the third port exceeding a presetthreshold.

In some embodiments, the transmit/receive switch may be configured sothat the third transistor turns on in response to a detection that thelevel of an RF signal at the third RF port exceeds a preset threshold.

Pursuant to still further embodiments of the present invention, methodsof protecting an amplifier that is coupled to an antenna through atransmit/receive selection device from an overdrive condition areprovided. Pursuant to these methods, a determination may be made that alevel of an RF signal at an RF port of the transmit/receive selectiondevice exceeds a preset threshold. The transmit/receive selection devicemay be configured to reflect RF energy received at an input thereof inresponse to determining that the level of the RF signal at the RF portexceeds the preset threshold.

In some embodiments, the method may also include taking a sample of theRF signal and passing the sample to an RF detector. A capacitor dividernetwork may be used to extract the sample of the RF signal. In someembodiments, the capacitor divider network may include a variablecapacitor. In some embodiments, the transmit/receive selection devicemay be a transmit/receive switch. In some embodiments, the presetthreshold may be a first preset threshold when the transmit/receiveswitch operates in a transmit mode, and the preset threshold may be asecond preset threshold when the transmit/receive switch operates in areceive mode, where the second preset threshold is different from thefirst preset threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a conventional time divisionduplex RF communications system.

FIG. 2 is a schematic block diagram of a conventional time divisionduplex RF communications system that includes an integrated hard limitercircuit.

FIG. 3 is a schematic block diagram of a time division duplex RFcommunications system that includes a transmit/receive circuit accordingto embodiments of the present invention.

FIG. 4 is a schematic circuit diagram of an example embodiment of the RFcommunications system of FIG. 3.

FIG. 5 is a graph illustrating the simulated output power as a functionof input power for the transmit/receive switch included in the RFcommunications system of FIG. 4.

FIG. 6 is a graph of the measured output power of the transmit/receiveswitch of the RF communications system of FIG. 4 as a function offrequency and the power of the input signal.

FIG. 7 is a schematic block diagram of a time division duplex RFcommunications system according to further embodiments of the presentinvention that includes a circulator.

FIG. 8 is a schematic block diagram of a time division duplex RFcommunications system according to further embodiments of the presentinvention that includes a different single-pole double-throwtransmit/receive switch implementation.

FIG. 9 is a schematic block diagram of an RF communications systemaccording to still further embodiments of the present invention.

FIG. 10 is a schematic layout diagram of a MMIC RF communications systemaccording to embodiments of the present invention.

FIG. 11 is a flow chart illustrating a method of protecting an amplifierfrom an overdrive condition according to additional embodiments of thepresent invention.

FIG. 12 is a schematic block diagram of an RF communications systemaccording to yet additional embodiments of the present invention.

DETAILED DESCRIPTION

FIG. 1 is a simplified block diagram of a time division duplexed RFcommunications system 10 that includes a low noise amplifier. In a timedivision duplexed RF communications system, the system may transmit RFsignals during some time periods (called time “slots”) and receive RFsignals during other time slots.

As shown in FIG. 1, the RF communications system 10 includes an RFtransceiver 20 that has a transmit port 22 and a receive port 24. Thetransmit port 22 of the RF transceiver 20 is coupled to a high poweramplifier (HPA) 30, which may be used to significantly increase thepower level of an RF signal output by RF transceiver 20 at port 22.Similarly, a low noise amplifier (LNA) 40 is coupled to the receive port24 of the RF transceiver 20. A transmit/receive switch 50 is providedthat selectively connects the high power amplifier 30 and the low noiseamplifier 40 to an antenna 60.

The time division duplexed RF communications system 10 may operate asfollows. During transmit mode time slots, the RF transceiver 20 mayreceive, for example, baseband data from baseband equipment (not shown)and may generate an RF signal that includes the data embedded therein.This RF signal is output from port 22 of RF transceiver 20 to the highpower amplifier 30, which increases the magnitude of the RF signal. Theamplified RF signal output by the high power amplifier 30 passes throughthe transmit/receive switch 50 to the antenna 60 that radiates theamplified RF signal into free space. The RF transceiver 20 and thetransmit/receive switch 50 may be controlled so that thetransmit/receive switch 50 connects the high power amplifier 30 to theantenna 60 during the transmit time slots of the time division duplexaccess scheme and connects the low noise amplifier 40 to the antenna 60during the receive time slots.

During receive mode time slots, RF signals are received by the antenna60 and passed by the transmit/receive switch 50 to the low noiseamplifier 40. The low noise amplifier 40 is designed to amplify RFsignals having relatively low power levels, as the RF signals arereceived over an air interface and hence subject to significant freespace attenuation. Consequently, the low noise amplifier 40 can besubject to damage from any higher power RF signals that pass through theantenna 60 thereto. For example, RF signals that are transmitted byother nearby transceivers may be received at the antenna 60 and passedthrough the transmit/receive switch 50 to the low noise amplifier 40.Due to the relatively low free space loss for close-by transmittersand/or the additive effect of multiple transmitters, the received RFsignal may be large enough to damage the low noise amplifier 40. Asanother example, if the antenna 60 is pointed towards a conductivesurface, the RF signals transmitted by the RF transceiver 20 through theantenna 60 may be reflected back into the antenna 60 where they mayagain pass to the low noise amplifier 40. If the power level of these RFsignals is sufficiently large, the low noise amplifier 40 may be damagedor even destroyed.

Hard limiter circuits may be used to protect RF electronic devices suchas the low noise amplifier 40 of FIG. 1 from potentially damaging RFsignal levels. Hard limiter circuits may be implemented, for example, byproviding one or more protection diodes that are coupled between the RFsignal path and ground. These hard limiter circuits may limit thecurrent delivered to the low noise amplifier (or other RF electronicdevice) to prevent damage thereto. The hard limiter circuit may beimplemented on the same chip as the RF electronic device that itprotects, and an additional external (off-chip) protection circuit maybe provided to supplement the on-chip hard limiter circuit. The externalprotection circuit may be placed, for example, between an antenna andthe chip that includes the RF component to be protected so that theexternal protection circuit will reduce the magnitude of an RF signalreceived at the antenna and the on-chip protection circuit may furtherreduce the magnitude of the received RF signal.

FIG. 2 is a schematic block diagram of a conventional RF communicationssystem 10′ that includes a transmit/receive circuit for a time divisionduplex system that has a built in hard limiter circuit that providesoverdrive protection for a low noise amplifier. As shown in FIG. 2, theRF communications system 10′ may be identical to the RF communicationssystem 10, except that the RF communications system 10′ further includesa hard limiter circuit 70 that is interposed between thetransmit/receive switch 50 and the low noise amplifier 40. The hardlimiter circuit 70 is configured to attenuate any RF signals received atan input 72 thereto, thereby protecting the low noise amplifier 40 fromunintended high power RF signals that may be received at the antenna 60.

Unfortunately, the above-described conventional hard limiter circuit 70may negatively impact the performance of the RF communications system 10by decreasing output power, gain and/or system sensitivity, and/or byincreasing the noise figure. For example, the above-described hardlimiter circuit 70 may include protection diodes that may add loss andparasitic capacitance to the RF circuit path, which increases the noisefigure of the low noise amplifier 40 and degrades system performance.

Pursuant to some embodiments of the present invention, monolithicmicrowave integrated circuits are provided that include at least onetransistor amplifier and a protection circuit that may protect theamplifier from unpredictable and unintended high power RF signals. Insome embodiments, the monolithic microwave integrated circuit maycomprise a transmit/receive circuit for a time division duplex RFcommunications system. The transmit/receive circuit may include a highpower amplifier, a low noise amplifier and a transmit/receive selectiondevice such as a switch or a circulator that is used to selectivelyconnect one of the high power amplifier and the low noise amplifier toan antenna. The transmit/receive selection device may be configured toprotect upstream components of the RF communications system such as thehigh power amplifier and/or the low noise amplifier from high power RFsignals that are unintentionally received by the antenna. Since thetransmit/receive selection device is already a necessary part of manytime division duplex RF communications systems, adding the overdriveprotection function to the transmit/receive selection device may not addsignificant loss or otherwise degrade system performance.

In some embodiments, the monolithic microwave integrated circuit(“MMIC”) may be configured to sample an RF signal that is input thereto(e.g., from an antenna) in order to detect a power level of the RFsignal (where the “detection” may merely be determining whether or notthe power level of the RF signal is above or below a predefinedthreshold). If the power level of the received RF signal exceeds thepredefined threshold, the configuration of the transmit/receiveselection device may be modified so that much of the RF energy input tothe MMIC device is reflected back out to the antenna or is otherwiserouted away from the transmit and receive paths that connect to anexternal RF transceiver. In one example implementation, the MMICincludes a single-pole double-throw transmit/receive switch. A node ofthis switch that connects a common RF port of the switch to the othertwo RF ports may be selectively connected to ground when a power levelof the received RF signal exceeds the predefined threshold level. Whenthe switch is short-circuited in this fashion, the common port willreflect a large portion of the RF energy that is input thereto, therebyprotecting upstream components. Since the RF communications system willtypically be designed to pass higher power RF signals along the transmitpath than the receive path, the RF communications system may beconfigured to apply different thresholds for triggering the protectionfunction depending upon whether the system is operating in transmit orreceive mode. While the power level of the RF signal may be detected insome embodiments, it will be appreciated that other levels of the RFsignal (e.g., a voltage level, a current level) may be detected insteadof, or in addition to, a power level.

Since the protection circuits according to embodiments of the presentinvention do not require the use of high power handling, low loss diodeslike conventional hard limiter circuits, the protection circuit can beimplemented on the same circuit substrate as the amplifier(s) and thetransmit/receive switch. In some embodiments, the amplifiers, thetransmit/receive switch and the protection circuits may be implementedusing gallium nitride based transistors. It can be difficult toimplement high power handling, low loss diodes on the same circuitsubstrate on which high power gallium nitride based transistoramplifiers are implemented. Pursuant to embodiments of the presentinvention, all of these components may be implemented on a singlemonolithic microwave integrated circuit, thereby reducing the size andcost of the RF communications system.

In some embodiments, the transmit/receive switch may be implemented asan RF power detector in combination with a single-pole double-throwswitch that has first through third RF ports and further includes anextra “protection” transistor that is coupled between the third RF portof the transmit/receive switch and a reference voltage such as ground.During normal operation, the protection transistor may be turned off(non-conducting) and the single-pole double-throw switch may connect thethird RF port to one or the other of the first RF port (which connectsto a high power amplifier) and the second RF port (which connects to alow noise amplifier) based on the values of one or more control signalsthat are provided to the transmit/receive switch. If the RF powerdetector detects that the power level of the RF signal that is presentat the common port of the switch is above a predefined threshold, theprotection transistor may be turned on, and the short circuit to groundacts to reflect much of the RF energy that is input to thetransmit/receive switch at the common port, thereby reducing the amountof RF energy that is passed to either the low-noise amplifier and/or thehigh power amplifier.

In some embodiments, monolithic microwave integrated circuits areprovided that include a substrate, a transmit/receive selection devicethat is formed on the substrate, a high power amplifier formed on thesubstrate and coupled to a first RF port of the transmit/receiveselection device, a low noise amplifier formed on the substrate andcoupled to a second RF port of the transmit/receive selection device andan overdrive protection circuit that is coupled to a first control portof the transmit/receive selection device.

In other embodiments, circuits are provided that include a substratehaving a switch formed thereon. The switch includes first through thirdRF ports, a first RF transmission path coupled between the third RF portand the first RF port, a second RF transmission path coupled between thethird RF port and the second RF port, a first transistor that is coupledbetween the first RF transmission path and a first reference voltage, asecond transistor that is coupled between the second RF transmissionpath and a second reference voltage, a third transistor that is coupledbetween a first node that connects the first and second RF transmissionpaths and a third reference voltage, and an overdrive protection circuitthat is configured to turn on the third transistor in response to apower level of an RF signal at the third port exceeding a presetthreshold.

In still other embodiments, transmit/receive switches are provided thatinclude a first RF port, a second RF port, a third RF port that may beselectively connected to one of the first RF port and the second RFport, a first RF transmission path coupled between the third RF port andthe first RF port, a second RF transmission path coupled between thethird RF port and the second RF port, a first transistor that is coupledbetween the first RF transmission path and a first reference voltage, asecond transistor that is coupled between the second RF transmissionpath and a second reference voltage, and a third transistor that iscoupled between a first node that connects the first and second RFtransmission paths and a third reference voltage.

The RF devices according to embodiments of the present invention mayhelp mitigate all types of overload conditions including both highcurrent levels and overvoltage conditions. Example embodiments of theswitches, transmit/receive circuits and monolithic microwave integratedcircuits according to embodiments of the present invention will now bedescribed in greater detail with reference FIGS. 3-11.

FIG. 3 is a schematic block diagram of a time division duplex RFcommunications system 100 according to embodiments of the presentinvention. As shown in FIG. 3, the RF communications system 100 includesan RF transceiver 120 that has a transmit port 122 and a receive port124. The RF transceiver 120 may be connected to baseband equipment (notshown). The RF transceiver 120 may output RF signals that are to betransmitted through the transmit port 122. The transmit port 122 of theRF transceiver 120 is coupled to a high power amplifier (HPA) 130, whichmay be used to significantly increase the power level of the RF signaloutput by transceiver 120. Similarly, a low noise amplifier (LNA) 140 iscoupled to the receive port 124 of the transceiver 120. Atransmit/receive switch 150 according to embodiments of the presentinvention is provided that includes first, second and third RF ports152, 154, 156. The high power amplifier 130 is coupled to the first RFport 152, the low noise amplifier 140 is coupled to the second RF port154, and an antenna 160 is coupled to the third (common) RF port 156. Itwill be appreciated that the high power amplifier 130, the low noiseamplifier 140 and the antenna 160 may be directly connected to therespective RF ports 152, 154, 156 or may be coupled to the RF ports 152,154, 156 through intervening elements.

Additionally, the RF communications system 100 includes a protectioncircuit 170. The protection circuit 170 includes a sampling circuit 172,an RF detector 180 and a control signal generation circuit 190. The RFdetector 180 may be connected to the RF transmission path 158 from theantenna 160 to the third RF port 156 of the transmit/receive switch 150via the sampling circuit 172. The sampling circuit 172 may sample asmall portion of the RF signal that is present on the RF transmissionpath 158 and pass the sampled RF signal to the RF detector 180. The RFdetector 180 generates an output signal based on a level of the sampledRF signal, and passes this output signal to a control signal generationcircuit 190. The control signal generation circuit 190 generates acontrol signal in response to the output signal from the RF detector 180and uses this control signal to control operation of thetransmit/receive switch 150. In particular, the control signal maydisable the transmit/receive switch 150 when the sampled RF signalexceeds a predetermined level in order to block the flow of RF signalsto the high power amplifier 130 and the low noise amplifier 140.Notably, the hard limiter used in conventional protection circuits maybe omitted in the protection circuit 170 according to embodiments of thepresent invention.

FIG. 4 is a schematic circuit diagram illustrating one exampleimplementation of the RF communications system 100 of FIG. 3. As shownin FIG. 4, the transmit/receive switch 150 may be implemented usingvarious transistors, transmission lines, resistors and inductors. In thedepicted embodiment, the transmit/receive switch 150 includes first,second and third transistors Q1, Q2, Q3, first and second transmissionline segments T1, T2, first, second and third resistors R1, R2, R3 andan inductor L1.

The inductor L1 is coupled between the third RF port 156 and a firstnode N1. The inductor L1 is a conventional part of a single-poledouble-throw switch and may be implemented, for example, as a spiralconductive trace on a substrate. The first transmission line segment T1connects node N1 to the first RF port 152 and the second transmissionline segment T2 connects node N1 to the second RF port 154. The firsttransistor Q1 has a first source/drain region that is connected to thefirst transmission line T1 and a second source/drain region that isconnected to a first reference voltage such as ground. The gate of thefirst transistor Q1 is connected to a first control input Control 1through the resistor R1. The second transistor Q2 has a firstsource/drain region that is connected to the second transmission line T2and a second source/drain region that is connected to a second referencevoltage such as ground. The gate of the second transistor Q2 isconnected to a second control input Control 2 through the resistor R2.The third transistor Q3 is connected between node N1 and a thirdreference voltage such as ground. The gate of the third transistor Q3 isconnected to an output of the protection circuit 170 through theresistor R3. The first transistor Q1 and the second transistor Q2 mayeach be about a quarter wavelength (where the wavelength is thewavelength corresponding to the operating bandwidth of the device) fromnode N1.

The sampling circuit 172 includes a capacitor C1 and a variablecapacitor C2. The capacitor C1 couples a small portion of any RF signalthat is present on the transmission path 158 and feeds the sampledportion to the RF detector 180 via a transmission path 174. The variablecapacitor C2 can be coupled between the transmission path 174 and areference voltage such as ground. The variable capacitor C2 may have,for example, two capacitance settings, one of which may be used when thetransmit/receive switch 150 is set for transmission of RF signals andthe other of which is used when the transmit/receive switch 150 is setfor reception of RF signals. This allows the protection circuit 170 tohave multiple trigger points so that the protection circuit 170 may beactivated at a first RF power level during transmit operations and at asecond, different RF power level during receive operations.

The RF detector 180 includes first and second diodes D1, D2 and acapacitor C3. The control signal generation circuit 190 includes a pairof transistors Q4, Q5 that are coupled in series between a voltagesource VSS (which may output a negative voltage such as, for example,−20 volts DC) and a reference voltage (e.g., ground) and a diode D3. Inthe depicted embodiment, the control signal generation circuit 190 isimplemented as an inverting DC amplifier. It will be appreciated,however, that a wide variety of different control signal generationcircuits 190 could be used, and that in some embodiments, the controlsignal generation circuit 190 could be omitted entirely.

The circuit of FIG. 4 may operate as follows. When the RF communicationssystem 100 is operating in receive mode, control signals are applied tocontrol ports Control 2 and Control 3 that turn off transistor Q2 andtransistor Q3, respectively. A control signal is applied to control portControl 1 that turns on transistor Q1. With these control signals, thetransmit/receive switch 150 will route RF signals received at the thirdRF port 156 through to the second RF port 154 to the low noise amplifier140, since the short circuit to ground through transistor Q1 willprevent RF energy from passing to the first RF port 152. Assuming thatthe antenna 160 is not receiving an unintended signal, then the powerlevel of the received RF signal that is passed to the third RF port 156is relatively small. Consequently, the sample of the RF signal that iscoupled onto transmission path 174 by the sampling circuit 172 may havea low power level, and hence the voltage at node N2 (i.e., the voltageat the gate of transistor Q4) is slightly higher than VSS and transistorQ4 is forward biased (conducting). As such, the source of transistor Q5is at about VSS (specifically, VSS minus the drain-source voltage oftransistor Q4, which is small due to transistor Q4 being in its lowresistance or “on” state). The source of transistor Q5 is coupled to thegate of transistor Q3 through resistor R3. When the source of transistorQ5 is at about VSS (which is a negative voltage), transistor Q3 will beturned off, and the transmit/receive switch 150 will operate like aconventional single-pole double-throw switch.

If an unintended RF signal having a high power level is received throughantenna 160, the sample of this RF signal that is passed by the samplingcircuit 172 to the RF detector 180 will have a larger power level. TheRF detector 180 is implemented as a diode peak detector circuit thatincludes the small diodes D1, D2 and a capacitor C3 that convert thesample of the RF signal into a direct current (“DC”) voltage. In theparticular implementation of the RF detector 180 illustrated in FIG. 4,the circuit is configured to route DC current from voltage supply VSSaway from the gate of transistor Q4 and so that output of the peakdetector 180 will become more negative as the magnitude of the sample ofthe RF signal increases. As the voltage level at the gate of transistorQ4 drops, there is little change in the voltage applied to control portControl 3 so long as transistor Q4 remains forward biased. Once thevoltage at the gate of transistor Q4 becomes sufficiently negative,transistor Q4 turns off, and the source of transistor Q5 goes to ground(0 volts). When this occurs, transistor Q3 turns on, connecting node N1to ground. This in turn shorts out the transmit/receive switch 150, andhence most of the RF energy that passes from the antenna 160 toward thetransmit/receive switch 150 is reflected by the third RF port 156,thereby reducing the amount of RF energy that flows to the low noiseamplifier 140 (as well as to the high power amplifier 130).

When the RF communications system 100 is operating in transmit mode, thesystem will operate in the same manner discussed above, except thatunder normal operations the transmit/receive switch will connect thefirst RF port 152 to the third RF port 156 so that the RF signals outputfrom the high power amplifier 130 are passed to the antenna 160. Inaddition, the variable capacitor C2 may be set to a different level toadjust the threshold at which the overdrive protection circuit 170 turnson the “protection transistor” Q3 in order to short circuit thetransmit/receive switch 150. This allows the much larger RF signal thatis output from the high power amplifier 130 to pass through thetransmit/receive switch 150 without turning on the protection transistorQ3. A control circuit (not shown), which may be as simple as a singletransistor, may be used to adjust the setting of the variable capacitorC2 as the transmit/receive switch 150 toggles between transmit andreceive modes.

In some embodiments, the control circuit may be configured so thatControl 1 and Control 2 are set to zero volts when the protectioncircuit 170 turns on the “protection transistor” Q3 in order to shortcircuit the transmit/receive switch 150. This may, in some cases,further improve the isolation between RF ports 152 and 154 with respectto RF port 156.

As shown in FIG. 4, the sampling circuit 172 connects to thetransmission path 158 at a node N3. According to some embodiments, aminimum electrical distance may be provided between node N3 and node N4of at least 1/12 of a wavelength of the RF signal (where the wavelengthof the RF signal is based on the center frequency of the RF signal). Theprovision of the spiral inductor L1 may help ensure that this minimumdistance is met. Ensuring that a minimum electrical distance is providedbetween nodes N3 and N4 may help ensure that the impedance at thesampling point is not reduced when transistor Q3 changes state, whichwould lower the sampled voltage and possibly cause the circuit to notproperly operate.

While the above description dikusses the protection circuit 170 beingactivated at various RF power levels (e.g., when a first RF power levelis exceeded during transmit operation and a second, lower, RF powerlevel during receive operations), it will be appreciated that theprotection circuit may detect a parameter other than power. For example,in the embodiment of FIG. 4, the protection circuit is triggered basedon a voltage at the sampling point, but the voltage at the samplingpoint corresponds to a power level since the transmission line impedanceis fixed. Thus, herein circuit 180 is referred to as an “RF detector” asit may detect any appropriate characteristic of the sampled RF signalsuch as a voltage level, a power level, etc. that is used to trigger theprotection circuit to change states.

FIG. 5 is a graph illustrating the simulated output power of thetransmit/receive switch 150 included in the RF communications system ofFIG. 4 as a function of input power. In particular, curve 200illustrates the RF power level at the first RF port 152 (which connectsto high power amplifier 130) as a function of the RF power level at thethird RF port 156 when the system is operating in transmit mode. Asshown by curve 200, the power level of the RF signal at the first RFport 152 generally tracks the power of the RF signal at the third RFport 156 (the power is about 1 dB less due to losses within thetransmit/receive switch 150) until the RF power at the third RF portreaches about 45.5 dBm. At this power level at the third RF port 156,the power at the first RF port 152 drops rapidly by nearly 10 dB, as theprotection transistor Q3 turns on and much of the RF energy (nearly 90%in this example) is reflected back towards the antenna 160.

Curve 210 illustrates the RF power level at the second RF port 154(which connects to the low noise amplifier 140) as a function of the RFpower level at the third RF port 156 when the system is operating intransmit mode. As shown, curve 210 has the exact same shape as curve200, but the power level is reduced by about 18 dB due to the isolationin the transmit/receive switch 150. As with curve 200, for power levelsabove 45.5 dBm, nearly 10 dB of additional isolation is achieved sincethe protection transistor Q3 turns on at this threshold and much of theRF energy (nearly 90% in this example) is reflected back towards theantenna 160. Curves 220 and 230 show the characteristics when thetransmit/receive switch 150 is operating in receive mode. It can be seenthat the exact same performance is achieved in receive mode. In FIG. 5,the points labelled 250 represent the threshold level where theprotection circuit 170 starts operating when the system is operating inreceive mode, while the points labelled 260 represent the thresholdlevel where the protection circuit 170 starts operating when the systemis operating in transmit mode. It should be noted that the points 250are at a power level (at port 156) of 37.5 dBm. This trigger point isset to protect the more sensitive low noise amplifier 140 from damage.Thus, FIG. 5 illustrates that the protection circuit 170 can providealmost 10 dB reduction in the power level of a large RF signal that isinput at port 156 of the transmit/receive switch, thereby protectingboth the low noise amplifier 140 and the high power amplifier 130 fromdamage.

Referring again to FIG. 4, the transistor Q3 replaces a matchingcapacitor that would be included in the transmit/receive switch 150 iftransistor Q3 had not been added, and hence the addition of transistorQ3 has very little impact on the performance of the transmit/receiveswitch 150. This is in contrast to the hard limiter protection circuitincluded in the RF communications 10′ of FIG. 2, which introduces lossesand other performance degradations such as, for example, decreasedoutput power, lower gain, reduced efficiency, increased systemsensitivity and/or an increased noise figure. Additionally, the transmitreceive circuits according to embodiments of the present inventionprovide protection to both the low noise amplifier 140 and the highpower amplifier 130, both of which are susceptible to damage. Forinstance, the high power amplifier 130 may be subject to damage if aconductive surface (e.g., a stabilizer on an airplane) is moved in frontof the RF communications system, as such an object can reflect much ofthe transmitted RF energy back through the antenna 160 toward thetransmit/receive switch 150. Moreover, the protection circuit 170 may bereadily implemented on the same circuit substrate as one or more of thetransmit/receive switch 150, the high power amplifier 130 and the lownoise amplifier 140. Thus, in some embodiments of the present invention,a monolithic microwave integrated transmit/receive circuit may beprovided that includes all four of the protection circuit 170, thetransmit/receive switch 150, the high power amplifier 130 and the lownoise amplifier 140 implemented in a single integrated circuit chip.

FIG. 6 is a graph of the measured output power at RF port 154 of thetransmit/receive switch of FIG. 4 as a function of both the frequencyand the power level of the RF signal at RF port 156 when thetransmit/receive switch 150 is operating in receive mode. As shown inFIG. 6, at an input power level of 36 dBm, the transmit/receive switch150 operates similar to a conventional transmit/receive switch, and theRF power level at RF port 154 is about 34.5 dBm, representing a loss ofabout 1.5 dB in the transmit/receive switch 150. At input RF powerlevels of 38 dBm and 39 dBm, the protection circuit 170 oftransmit/receive switch 150 operates normally and the RF power level atRF port 154 is reduced by about 9 dBm from the power level at RF port156 in each case, although the response is not constant with frequency.The variation as a function of frequency may be caused by measurementlimitations rather than a circuit performance issue. When the switch 150enters protection mode, the reflected energy returns to the test systempower amplifier which may cause the output power of the amplifier tochange (due to this non-optimal load condition on the PA), and alsoreduces measurement accuracy of the incident power. At an input RF powerlevel of 37 dBm at RF port 156, the protection circuit 170 starts tooperate and transient performance is seen as protection is provided atsome frequencies, but not at others.

FIG. 7 is a schematic block diagram of an RF communications system 300according to further embodiments of the present invention. As shown inFIG. 7, the RF communications system 300 is very similar to the RFcommunications system 100 discussed above, except that thetransmit/receive switch 150 of RF communications system 100 is replacedin RF communications system 300 with a circulator 350. As known to thoseof skill in the art, a circulator is an RF circuit that typically hasthree or four ports that is designed so that RF energy entering thedevice at one port is transmitted only to the next port in a directionof rotation of the circulator. The circulator 350 that is included inthe RF communications system 300 may be a non-conventional circulatorthat is designed to reflect RF energy received at one of the RF portsthereof in response to an RF power level present at the port exceeding apredetermined threshold.

As shown in FIG. 7, the circulator 350 includes four RF ports 352, 354,356 and 358. RF energy that is input to RF port 352 is output through RFport 356. RF energy that is input to RF port 356 is output through RFport 354. RF energy that is input to RF port 354 is output through RFport 358. In normal operations, no RF energy will be input to RF port354, but in the event that this occurs, the RF circulator 350 will passsuch RF energy to port 358 where it will be terminated in a dissipativeload 351.

The RF communications system 300 may operate essentially identically tothe RF communications system 100 discussed above, as the circulator 350will functionally perform equivalent operations to the transmit/receiveswitch 150 of RF communications system 100. The circulator 350 mayinclude a reflection circuit (not shown) that is configured tosubstantially prevent RF energy input at RF port 356 from flowing outthrough RF port 354 toward the low noise amplifier 140. As a result,most of the RF energy input at RF port 356 will not flow through RF port354, but instead will continue passing through the circulator 350 andexit the circulator 350 at RF port 358, where it is terminated in adissipative load 351. In an alternative embodiment, a three portcirculator may be used that does not include RF port 358 (or thedissipative load 351). In this embodiment, a reflection circuit (notshown) may be provided at RF port 356 that, when activated, isconfigured to substantially prevent RF energy input at antenna 160 frompassing into the circulator 350. This reflection circuit may beimplemented, for example, as a circuit that debiases the magneticferrite of the circulator 350 to make the circulator 350 lossy in thebandwidth of the offending (undesired) RF signal. In each of the aboveexample embodiments, a control port that is coupled to the protectioncircuit 170 may control the reflection circuit to activate thereflection circuit when the protection circuit 170 determines that theRF power level at RF port 356 exceeds a predefined threshold.

FIG. 8 is a schematic block diagram of an RF communications system 400according to further embodiments of the present invention that includesa different transmit/receive switch implementation that includes asingle-pole double-throw transmit/receive switch 450. The single-poledouble-throw switch 450 illustrated in FIG. 8 is similar to thesingle-pole double-throw transmit/receive switch shown in FIG. 1 of U.S.Pat. No. 8,421,122, the entire content of which is incorporated hereinby reference. However, as compared to the switch disclosed in U.S. Pat.No. 8,421,122, the single-pole double-throw switch 450 in FIG. 8includes an additional transistor Q14 which is coupled between node N1and a reference voltage (e.g., ground).

Referring to FIG. 8, the transmit/receive switch 450 includes a firstcontrol input Control 1, a complementary control input Control 2, andfirst through third RF ports 452, 454, 456. The transmit/receive switch450 further includes five gallium nitride based HEMT transistors Q10 toQ14. The second control input Control 2 is connected through resistorsR2 and R3 to gates of transistors Q11 and Q12. The first control inputControl 1 is connected through resistors R1 and R4 to gates oftransistors Q10 and Q13. The resistors R1 to R4 at the gates oftransistors Q10 to Q13 may have relatively large resistances in someembodiments, such as, for example, resistances of about 10,000 ohms RFport 452 is connected through an inductance L2 to the sixth node N6. RFport 454 is connected through an inductance L3 to the fifth node N5. RFport 456 is connected through inductance L1 to the first node N1. Theinductors L1 to L3 may include spiral inductor networks that aredesigned to match the switch on the monolithic die to 50 ohms impedance.

The RF communications system further includes the above-describedprotection circuit 170. As shown in FIG. 8, the output of the protectioncircuit is coupled to the gate of transistor Q14. The protection circuitmay be configured to turn transistor Q14 on in response to detectingthat the RF power level at port 456 exceeds a predefined threshold. Asthe RF communications system 400 may operate in essentially the samemanner as the RF communications system 100 with the exception that adifferent single-pole double-throw switch design is used, hence furtherdescription of the RF communications system 400 of FIG. 8 will beomitted.

It should be noted that in the embodiment of FIG. 8 the addedcapacitance of the protection transistor Q14 may force reduction insizes of the other transistors Q10-Q13, which may increase loss, anddecrease power handling and isolation. Accordingly, as shown in FIG. 12,in yet another embodiment, the protection transistor Q14 may be omittedand the control circuit configured to re-bias transistors Q10 and Q11with a large negative gate voltage while biasing transistors Q12 and Q13to zero volts. In doing so the protection will be added withoutsignificantly impacting the RF loss, isolation, or power handling.

FIG. 9 is a schematic block diagram of an RF communications system 500according to further embodiments of the present invention. The RFcommunications system 500 is very similar to the RF communicationssystem 100 discussed above, except that the transmit/receive switch 150of RF communications system 100 is replaced in RF communications system500 with a transmit/receive switch 550 that is designed to coupleexcessive RF energy that is present at RF port 156 to a load 551 asopposed to using a short circuit to reflect the RF energy back towardsthe antenna 160. While not shown in FIG. 9, control circuits are coupledbetween Control 3 and Control 1 and Control 2 that switch Control 1 andControl 2 to zero volts when the protection circuit 170 is activated.Given the similarities between RF communications system 500 and RFcommunications system 100, further description of RF communicationssystem 500 will be omitted.

As discussed above, the transmit/receive circuits according toembodiments of the present invention may be implemented as monolithicmicrowave integrated circuits that may include, for example, a highpower amplifier, a low noise amplifier, a transmit/receive switch and aprotection circuit that are all implemented on a single substrate as amonolithic integrated circuit. Such an implementation may haveadvantages in terms of size, cost and performance.

FIG. 10 is a schematic layout view of a monolithic microwave integratedcircuit implementation of a transmit/receive circuit 600 according toembodiments of the present invention. The monolithic microwaveintegrated circuit shown in FIG. 10 may comprise, for example, animplementation of the transmit/receive circuit portion of the RFcommunications system 100 of FIGS. 3-4.

As shown in FIG. 10, the transmit/receive circuit 600 includes aplurality of circuit elements that are formed on a monolithic substrate610. The substrate 610 may comprise, for example, a silicon carbide orsapphire substrate that has gallium nitride based epitaxial layersformed thereon, or other suitable epitaxial layers such as, for example,silicon carbide or gallium arsenide based epitaxial layers.Metallization patterns and dielectric layers may also be formed on thesubstrate 610 to implement the various circuit elements.

The circuit elements may include a high power amplifier 630, a low noiseamplifier 640 and a transmit/receive switch 650 and a protection circuit670. The high power amplifier 630 and the low noise amplifier 640 mayeach comprise a gallium nitride based transistor amplifier implementedusing a plurality of HEMT transistors. In the depicted embodiment, thehigh power amplifier 630 is illustrated as being a multi-stage amplifierthat has a first plurality of unit cell transistors that form a driveramplification stage 632 and a second plurality of unit cell transistors634 that form an output amplification stage. Each amplifier may alsoinclude one or more of an input impedance matching circuit, an outputimpedance matching circuit and/or inter-stage impedance matchingcircuit(s).

The transmit/receive switch 650 may comprise any of the transmit/receiveswitches according to embodiments of the present invention. For example,the transmit/receive switch 650 may comprise the single-poledouble-throw switch 150 of FIG. 4. The protection circuit 670 maycomprise any circuit that may control the transmit/receive switch 650 toredirect RF energy present at a common port thereof when a power levelof the RF energy exceeds one or more predefined thresholds. For example,the protection circuit 670 may comprise the protection circuit 170 shownin FIG. 4. The transmit/receive switch 650 further includes input/outputpads 612 and additional power and ground pads 614.

FIG. 11 is a flow chart illustrating a method of protecting an amplifierthat is coupled to an antenna through a transmit/receive selectiondevice from an overdrive condition according to certain embodiments ofthe present invention. As shown in FIG. 11, operations may begin with anRF signal that is at an input port of the transmit/receive selectiondevice being sampled (block 700). In some embodiments, thetransmit/receive selection device may be a single-pole double-throwswitch. The sample of the RF signal may be taken using, for example, acapacitor divider network. The sample of the RF signal may be passed toan RF detector (block 710). Next, a determination may be made as towhether or not a power level of an RF signal that is present at an RFport of the transmit/receive selection device exceeds a preset threshold(block 720). In response to this determination, the transmit/receiveselection device may be configured to reflect RF energy received at theinput port thereof (block 730).

It will be appreciated that the above embodiments are exemplary innature and are not intended to be limiting to the scope of the presentinvention. It will also be appreciated that numerous modifications maybe made to the above-described embodiments without departing from thescope of the present invention. As one simple example, the antenna 160illustrated in the above embodiments is schematically shown ascomprising a linear array of radiating elements such as are used inphased array antennas. it will be appreciated that the antenna 160 maycomprise any appropriate antenna, including dipole, patch, reflector,horn, loop and numerous other antennas, and may include a singleradiating element or surface or multiple radiating elements/surfaces,which may or may not be phase controlled. As another example, while theabove described embodiments are implemented as monolithic microwaveintegrated circuits, it will be appreciated that the various elements ofthe communications system may be implemented as separate integratedcircuits or other elements.

As another example, it will be appreciated that the present inventioncan be implemented in a wide variety of different switches, includingswitches other than transmit/receive switches and including switchesother than single-pole double-throw switches. For example, in otherembodiments, the systems could include a single-pole single-throwswitch, a triple pole switch, etc. Thus, it will be appreciated that thescope of the present invention is defined by the attached claims ratherthan by the example embodiments that are described in detail herein. Itwill also be appreciated that the above-described embodiments may becombined in any fashion. For example, the technique discussed withreference to FIG. 9 where the excess RF energy is passed to a load couldbe used in any of the other embodiments. It will likewise be appreciatedthat in some embodiments, the transmit/receive circuits may havedifferent numbers of control inputs than shown in the above exampleembodiments.

The transmit/receive switches according to embodiments of the presentinvention may exhibit low signal loss, low distortion, high powerhandling, low DC power dissipation, and good impedance matchcharacteristics. The transmit/receive switches disclosed herein may alsoprovide high levels of isolation between a non-selected port (i.e., theRF port connected to the high power amplifier when the switch operatesin receive mode, and the RF port connected to the low noise amplifierwhen the switch operates in transmit mode) over a wide frequency band.

Embodiments of the inventive concepts may be particularly well suitedfor use in connection with electronic circuits that are formed in GroupIII-nitride based semiconductor materials. As used herein, the term“Group III nitride” refers to those semiconducting compounds formedbetween nitrogen and the elements in Group III of the periodic table,usually aluminum (Al), gallium (Ga), and/or indium (In). The term alsorefers to ternary and quaternary compounds such as AlGaN and AlInGaN.These compounds all have empirical formulas in which one mole ofnitrogen is combined with a total of one mole of the Group III elements.

For example, many high power transistor amplifiers are formed usingGaN-based high electron mobility transistor (HEMT) devices. Suitablestructures for GaN-based HEMTs that may be used to implement theamplifiers included in embodiments of the present invention aredescribed, for example, in commonly assigned U.S. Publication No.2017/0271497, published Sep. 21, 2017, the disclosure of which isincorporated herein by reference in its entirety

The transistors included in the devices according to embodiments of theinventive concepts may include a semiconductor structure that is amultiple layer structure. For example, each transistor may be formed ona substrate which may be, for example, a silicon carbide (e.g., 4H-SiCor 6H-silicon carbide, sapphire, aluminum nitride, aluminum galliumnitride, gallium nitride, silicon, GaAs, LGO, ZnO, LAO, InP and thelike. A semiconductor structure may be formed on an upper surface of thesubstrate. The semiconductor structure may include at least a channellayer and a barrier layer, which may be sequentially stacked on thesubstrate and which may each comprise Group III nitride layers. Optionalbuffer, nucleation, strain balancing and/or transition layers may beprovided between the substrate and the channel layer. One or morecapping layers, such as silicon nitride layers, may be provided on anupper surface of the barrier layer.

The channel layer may comprise, for example, a Group III-nitride, suchas Al_(x)Ga_(1-x)N where 0≤x<1, provided that the energy of theconduction band edge of the channel layer is less than the energy of theconduction band edge of the barrier layer at the interface between thechannel and barrier layers. The channel layer may be undoped orunintentionally doped and may have a bandgap that is less than thebandgap of the barrier layer. The channel layer may have a largerelectron affinity than the barrier layer. In certain embodiments, thebarrier layer is AlN, AlInN, AlGaN or AlInGaN.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present invention. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a,” “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”“comprising,” “includes” and/or “including” when used herein, specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

It will be understood that when an element such as a layer, region orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present. Itwill also be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “lateral” or “vertical” may be used herein to describe arelationship of one element, layer or region to another element, layeror region as illustrated in the figures. It will be understood thatthese terms are intended to encompass different orientations of thedevice in addition to the orientation depicted in the figures.

Embodiments of the invention are described herein with reference tocross-section illustrations that are schematic illustrations ofidealized embodiments (and intermediate structures) of the invention.The thickness of layers and regions in the drawings may be exaggeratedfor clarity. Additionally, variations from the shapes of theillustrations as a result, for example, of manufacturing techniquesand/or tolerances, are to be expected. Thus, embodiments of theinvention should not be construed as limited to the particular shapes ofregions illustrated herein but are to include deviations in shapes thatresult, for example, from manufacturing.

In the drawings and specification, there have been disclosed typicalembodiments of the invention and, although specific terms are employed,they are used in a generic and descriptive sense only and not forpurposes of limitation, the scope of the invention being set forth inthe following claims.

What is claimed is:
 1. A monolithic microwave integrated circuit,comprising: a substrate; a transmit/receive selection device formed onthe substrate, the transmit/receive selection device including a firstradio frequency (“RF”) port, a second RF port, a third RF port and afirst control port; a high power amplifier formed on the substrate andcoupled to the first RF port of the transmit/receive selection device; alow noise amplifier formed on the substrate and coupled to the second RFport of the transmit/receive selection device; and a protection circuitcoupled to the first control port of the transmit/receive selectiondevice, wherein the protection circuit is configured to reconfigure thetransmit/receive selection device in response to detecting that a levelof an RF signal at the third RF port exceeds a first threshold duringtransmit operations and to reconfigure the transmit/receive selectiondevice in response to detecting that the level of the RF signal exceedsa second threshold during receive operations, the second threshold beingless than the first threshold.
 2. The monolithic microwave integratedcircuit of claim 1, wherein the transmit/receive selection devicecomprises a transmit/receive switch.
 3. The monolithic microwaveintegrated circuit of claim 2, further comprising a sampling circuitthat is configured to couple a sample of the RF signal received at thethird RF port and to provide the sample of the RF signal to theprotection circuit.
 4. The monolithic microwave integrated circuit ofclaim 3, wherein the sampling circuit is configurable to sample at leastone of a first amount and a second amount of the RF signal, wherein thesecond amount is greater than the first amount.
 5. The monolithicmicrowave integrated circuit of claim 3, wherein the sampling circuitfurther comprises a variable capacitor.
 6. The monolithic microwaveintegrated circuit of claim 1, wherein the protection circuit comprises:an RF detector; and a control signal generator circuit responsive to theRF detector, wherein an output of the control signal generator circuitis coupled to the first control port of the transmit/receive selectiondevice.
 7. The monolithic microwave integrated circuit of claim 2,wherein the transmit/receive switch comprises: a first transistor thatis coupled between a first reference voltage and a first RF transmissionpath that extends between the first RF port and the third RF port; asecond transistor that is coupled between a second reference voltage anda second RF transmission path that extends between the second RF portand the third RF port; and a third transistor that is coupled between athird reference voltage and a first node connecting the first RFtransmission path to the second RF transmission path.
 8. The monolithicmicrowave integrated circuit of claim 1, wherein the protection circuitis further configured to increase reflection of the RF signal receivedat the third RF port in response to detecting that the level of the RFsignal exceeds the first threshold during transmit operations and/orthat the level of the RF signal exceeds the second threshold duringreceive operations.
 9. The monolithic microwave integrated circuit ofclaim 7, wherein a distance between the first node and a node where asampling circuit is coupled along an RF path that extends between thetransmit/receive switch and an antenna is at least 1/12 of a wavelengthof an RF signal present at the third RF port.
 10. A circuit, comprising:a substrate having a switch formed thereon that includes: first radiofrequency (“RF”) port, a second RF port and a third RF port; a first RFtransmission path coupled to the first RF port; a second RF transmissionpath coupled to the second RF port; a first transistor that has aterminal coupled to the first RF transmission path; a second transistorthat has a terminal coupled to the second RF transmission path; a thirdtransistor that is coupled between a first node that connects the firstand second RF transmission paths and a third reference voltage; and aprotection circuit that is configured to turn on the third transistor inresponse to a level of an RF signal at the third RF port exceeding apreset threshold.
 11. The circuit of claim 10, further comprising asampling circuit that is configured to couple a sample of an RF signalreceived at the third RF port and to provide the sample of the RF signalto the protection circuit.
 12. The circuit of claim 11, wherein thesampling circuit comprises a capacitor voltage divider network that iscoupled to the third RF port.
 13. The circuit of claim 12, wherein thecapacitor voltage divider network includes a variable capacitor.
 14. Thecircuit of claim 10, wherein the protection circuit comprises: an RFdetector; and a control signal generator circuit responsive to the RFdetector, wherein an output of the control signal generator circuit iscoupled to a gate of the third transistor.
 15. The circuit of claim 14,wherein the control signal generator circuit comprises a transistor thatswitches between an on-state and an off-state when an output of the RFdetector reaches a threshold level.
 16. The circuit of claim 10,wherein: the first RF transmission path extends between the first nodeand the first RF port; the second RF transmission path extends betweenthe first node and the second RF port; the first transistor is coupledbetween the first RF transmission path and a first reference voltage;and the second transistor is coupled between the second RF transmissionpath and a second reference voltage.
 17. A transmit/receive switch,comprising: a first radio frequency (“RF”) port; a second RF port; athird RF port that may be selectively connected to one of the first RFport and the second RF port; a first RF transmission path coupledbetween the third RF port and the first RF port; a second RFtransmission path coupled between the third RF port and the second RFport; a first transistor that is coupled between the first RFtransmission path and a first reference voltage; a second transistorthat is coupled between the second RF transmission path and a secondreference voltage; and a third transistor that is coupled between afirst node that connects the first and second RF transmission paths anda third reference voltage.
 18. The transmit/receive switch of claim 17,further comprising a protection circuit that is configured to turn onthe third transistor in response to a level of an RF signal at the thirdRF port exceeding a preset threshold.
 19. The transmit/receive switch ofclaim 17, wherein the transmit/receive switch is configured so that thethird transistor turns on in response to a detection that a level of anRF signal at the third RF port exceeds a preset threshold.